Erlang Calculation

Hi, I am a freshman in college taking up telecommunications and I have a tough time figuring how to solve the following problem :

A junction network has four local exchanges (A,B,C,D) and it may also have a tandem exchange (T). During the busy hour, each local exchange originates 2.0 E (Erlang) of traffic to each others and the required grade-of-service (GoS) is 0.02. Determine the total number of one-way junctions required in the network if :
i. Direct junctions only are provided (i.e. no tandem exchange)
ii. All junction calls are routed through a tandem exchange
iii. Automatic alternative routing is used and there are four direct junctions between each pair of local exchanges (two in each direction), with overflow traffic routed via the tandem exchange

Please help me. I did some research and I still could not figure it out. What are the formulas used in solving these questions. Appreciate it if someone can guide me. Thank you.